The invention relates generally to photolithography, and more specifically to step and scan lithography.
The fabrication of integrated circuits entails a number of photolithography steps wherein finely detailed patterns corresponding to the features in different layers of the chip are photographically replicated on a photoresist layer on the surface of a wafer of silicon or other semiconductor material. Depending on the type of photoresist, the exposed parts or the unexposed parts are rendered (or remain) removable, so that after suitable treatment a positive or negative pattern of photoresist corresponding to the projected pattern remains. A typical process may entail up to 20 or more such steps.
The projection process utilizes a set of reticles, with each reticle carrying a pattern corresponding to one layer on one chip. The reticle is itself fabricated in a lithographic process, and comprises a finely etched pattern of metal (such as chrome) on a glass or quartz substrate. The reticle can be used directly or can be copied many times onto a larger plate to produce a mask that corresponds to the pattern for up to the entire wafer surface.
An early approach was contact alignment, where the mask was brought into intimate contact with the photoresist layer, and ultraviolet light from a uniform collimated source illuminated the entire mask. Contact aligners gave way to scanning projection aligners, as exemplified in the system shown in U.S. Pat. No. 4,068,947. The patent describes a system developed and marketed by the Perkin-Elmer Corporation under the trademark MICRALIGN.
In the Perkin-Elmer system, the mask image was projected onto the wafer surface using a mirror system for imaging at 1:1 (unit) magnification. References to reduction or demagnification imply a magnification less than 1.0.
The fixed optical system was characterized by an optical axis and first and second conjugate planes such that a point in one of the planes was imaged at a corresponding conjugate point in the other plane. A light source illuminated a selected area in the object plane of the imaging system where the imaging system possessed optimum imaging properties. In the particular type of imaging system disclosed, the selected region was an annular segment significantly off the optical axis. The reason for such a configuration is that the chord length of an arcuate slit could be extended and the field corrected for aberrations more easily than would be the case for the larger square or rectangular full field.
The wafer and mask were mounted to a carriage, with the mask in the object plane and the wafer surface in the image plane. The carriage was moved so that the illuminated region was scanned across the mask and the corresponding illuminated portion of the image was scanned across the wafer. By scanning in this manner, it was possible to obtain a high-quality image of a large mask on a large wafer, even though the mask and wafer were larger than the area of optimum imaging properties of the optical imaging system. In the particular apparatus, the mask and wafer were mounted to a rotating carriage.
A different approach (referred to as step and repeat) was to use a stepper to sequentially project a full image of the reticle onto each chip region on the wafer. This allowed better overlay alignment since each layer on each chip could be aligned individually. Moreover, the stepper allowed the use of reducing optics, which in turn allowed the reticle to be several times as large as the chip and thus provide improved resolution. However, as feature sizes continued to decrease and chip sizes continued to increase, the demands on a full field optical system became even more stringent, even when required to cover only one chip on the wafer.
This led to the step and scan lithography system, wherein each chip is exposed by scanning onto the wafer the demagnified image of a slit moving over the reticle. The concept is analogous to the MICRALIGN device, with the scanning repeatedly applied over the wafer. However, the MICRALIGN device had 1:1 magnification, which made it relatively straightforward to achieve synchronous scanning of the wafer and reticle relative to the optical system. For a reduction system, the wafer must move more slowly than the reticle, with the speed ratio being equal to the system demagnification (a number less than 1). At present, the only known commercially available step and scan system is marketed by SVGL (a successor to Perkin-Elmer's lithography business) under the trademark MICRASCAN. The synchronous scanning is achieved by using two interferometrically-controlled, air-bearing stages, driven by linear motors. The device presumably works for its intended purpose, but requires a large number of very precise moving parts, and thus is mechanically complex.
In addition to microcircuits, many of the above lithographic techniques have been applied to the fabrication of flat panel displays. These displays are made up of an array of pixels, each of which is a microcircuit, which are independently controllable by electronic circuits, so arbitrary images can be created. The contrast medium itself is often a liquid crystal. The pixel circuitry is built up on a substrate, often glass, by lithographic techniques. The liquid crystal is sandwiched between the circuitry plate and a transparent cover plate. These displays exist in a number of sizes, ranging from liquid crystal display watch faces to thin video displays for laptop computers. In the future, this technology may be applied to larger displays for high definition television.
This lithographic application differs in detail from that described above. Feature sizes are larger, and pattern overlay requirements are looser. However the display sizes may be much larger than the microcircuit chip sizes. These differences are reflected in the lithography tools used to make flat panel displays. Much larger image field sizes are used. Also, the looser image quality tolerances allow less, or no, optical reduction to be used (i.e., larger fractional values of magnification, or unit magnification). This can simplify the optical design, as well as keep the reticle size from getting too large. Indeed, reticle size considerations may dictate lithography machines in the future with greater than 1:1 magnification, rather than reduction optics. In some cases a single reticle may not cover all the features on a single level of the display, so several exposures, with different reticles, on different parts of the display may be required. Careful alignment of the edges of neighboring exposure field on the plate would then be required, to insure continuity of patterns across the boundary between fields, as well as proper exposure dose. This process is referred to as "butting". Both step and repeat and 1:1 magnification scanning systems have been used for flat panel display fabrication.